Passively-cooled spent nuclear fuel pool system

ABSTRACT

A passively-cooled spent nuclear fuel pool system in one embodiment includes a containment vessel comprising a thermally conductive shell and an annular reservoir surrounding the shell that holds a liquid coolant forming a heat sink. A spent fuel pool is disposed inside the containment vessel and includes a body of water in contact with a first peripheral sidewall of the fuel pool. At least one spent nuclear fuel rod submerged in the body of water heats the water. The first peripheral sidewall of the spent fuel pool is formed by a portion of the shell of the containment vessel adjacent to the fuel pool, thereby defining a shared common heat transfer wall. The heat transfer wall operates to transfer heat from the body of water in the spent fuel pool to the heat sink to cool the body of water. The heat transfer wall comprises metal in one embodiment.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 14/713,093 filed May 15, 2015, which claims the benefit of U.S.Provisional Patent Application No. 61/993,857 filed May 15, 2014. Thepresent application is also a continuation-in-part of U.S. patentapplication Ser. No. 14/620,465 filed Feb. 12, 2015 (now U.S. Pat. No.9,916,910), which in turn is a continuation-in-part of PCT/US2013/054973filed Aug. 14, 2013, which in turn claims the benefit of U.S.Provisional Patent Application No. 61/683,030 filed Aug. 14, 2012. Thepresent application is also a continuation-in-part of U.S. patentapplication Ser. No. 14/403,082 filed Nov. 21, 2014, (now U.S. Pat. No.9,786,393), which in turn is a national stage entry under 35 U.S.C. 371of PCT/US2013/042070 filed May 21, 2013, which in turn claims thebenefit of U.S. Provisional Patent Application No. 61/649,593 filed May21, 2012. The entireties of the aforementioned applications are allincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a passive system for cooling a spentnuclear fuel pool and a method of passively cooling a spent nuclear fuelpool.

BACKGROUND OF THE INVENTION

A spent fuel pool is a body of water inside a nuclear power plant's fuelstorage building which is typically about forty feet deep and which isequipped with fuel racks to store spent nuclear fuel that is dischargedfrom the reactor during refueling outages. The pool keeps the fuel in asafe underwater configuration absorbing the fuel's radiation and itsdecay heat. The decay heat deposited by the fuel into the pool's watermust be removed to prevent uncontrolled heat-up of the pool's water,which would result in undesirable evaporation of the fuel pool water. Inexisting nuclear plant design practice, the pool's water is cooled bypumping it though a heat exchanger, which is typically served by theplant's component cooling water, a closed loop purified water streamthat cools a variety of equipment in the plant and is in turn cooled bya natural source of water such as a lake, a river, or an ocean.

This conventional spent fuel pool cooling system has several drawbacks,the most notable of which is the dependence of the pool cooling on pumpsand motors to circulate water through both sides of the heat exchanger.During a power outage or some other event that disables the pumps andmotors, the water in the spent fuel pool will boil and evaporate whichcan lead to the fuel being exposed above the surface level of the poolwater. Another drawback is the continuous release of water vapor insidethe plant's fuel storage building which adds to the building's humidityand temperature affecting its habitability and increasing its HVACburden. The open pool also attracts dust and particulates from theambient air turning them into radioactive material which must besuctioned from the pool, filtered and collected for disposal ascontaminated waste.

Thus, a need exists for a system and method for cooling a spent nuclearfuel pool that does not rely on pumps and motors. A need also exists fora system and method for reducing the humidity inside of a nuclear powerplant fuel storage building. Furthermore, a need exists for a system andmethod for preventing dust and particulates from collecting in the spentfuel pool.

SUMMARY OF THE INVENTION

The present invention provides a passively-cooled spent nuclear fuelpool system and method therefor that overcomes the deficiencies of theforegoing existing arrangements. The approaches disclosed herein coolthe body of water in the spent fuel pool which is heated by radioactivefuel decay via evaporation, and also convection-conduction using anexternal heat sink relying on natural gravity-driven flow circulationpatterns as further described herein. These approaches do not rely onpumps or available electric power to effectively cool the spent fuelpool. The method of heat rejection therefore does not need any activecomponents or even any passive actuating devices to initiate the coolingprocesses. The heat rejection will automatically start and continue aslong as there is a heat source in the pool (i.e. spent fuel assemblies).

In one aspect of spent fuel pool cooling, a containment vessel iscircumscribed by an annular reservoir containing a liquid coolant suchas water. A portion of the cylindrical wall of the vessel is shared withand forms a common thermally-conductive wall with the spent fuel poolbetween the reservoir and spent fuel pool. In one embodiment, thisshared common wall is made of a high conductivity metal, such as withoutlimitation carbon or low alloy steel. This defines a conductive heattransfer wall between the spent fuel pool and the reservoir. The innersurface of the heat transfer wall (i.e. containment vessel) is in directcontact with and wetted by the spent fuel pool water. The coolant wateroutside the containment vessel in the annular reservoir is in contactwith the external surface of the heat transfer wall containment vessel.This configuration enables direct heat transfer from the heated spentfuel pool water to the lower temperature coolant reservoir viaconductive heat transfer through the metal heat transfer wall of thecontainment vessel to enhance cooling of the fuel pool. Other featuresare disclosed herein which further aid direct cooling of the body ofwater in the spent fuel pool via gravity driven thermal gradient inducedflow patterns.

In one form, a passively-cooled spent nuclear fuel pool system includes:a containment vessel comprising a thermally conductive cylindrical shellformed of metal; an annular reservoir surrounding the cylindrical shellof the containment vessel, the annular reservoir holding a liquidcoolant to form a heat sink; and a spent nuclear fuel pool disposedinside the containment vessel, the fuel pool comprising: a floor and afirst peripheral sidewall extending upwards from the floor thatcollectively define an interior cavity; a body of water disposed in theinterior cavity and having a surface level, the water being in contactwith the first peripheral sidewall; and at least one spent nuclear fuelrod submerged in the body of water that heats the body of water; whereinthe first peripheral sidewall of the fuel pool is formed by a portion ofthe cylindrical shell of the containment vessel adjacent to the spentfuel pool which defines a shared common heat transfer wall, the heattransfer wall operable to transfer heat from the body of water in thespent fuel pool to the heat sink for cooling the body of water.

In another form, a passively-cooled nuclear spent fuel pool systemincludes: a containment vessel comprising a thermally-conductivecylindrical shell formed of metal; an annular reservoir surrounding thecylindrical shell of the containment vessel, the annular reservoirholding a coolant that defines a heat sink; a spent nuclear fuel pooldisposed in the containment vessel, the fuel pool comprising: a floorand a first peripheral sidewall extending upwards from the floor thatcollectively define an interior cavity; a body of water disposed in theinterior cavity and having a surface level, at least one spent nuclearfuel rod submerged in the body of water that heats the water to formwater vapor via evaporation; a removable lid covering the spent nuclearfuel pool and configured to form a hermetically sealed vapor spacebetween the surface level of the body of water and the lid; a passiveheat exchange sub-system comprising an assembly of: a primary risersection fluidly coupled to the vapor space; at least one downcomerfluidly coupled to the primary riser section for receiving the watervapor from the primary riser section, the water vapor condensing withinthe at least one downcomer to form a condensed water vapor; and at leastone return conduit fluidly coupled to the at least one downcomer, the atleast one return conduit having an outlet located within the body ofliquid water for returning the condensed water vapor to the body ofliquid water; wherein the first peripheral sidewall of the fuel pool isformed by a portion of the cylindrical shell of the containment vesseladjacent to the spent fuel pool that defines a shared heat transferwall, the heat transfer wall operable to transfer heat from the body ofwater in the spent fuel pool to the heat sink for cooling the body ofwater.

A method for cooling a nuclear spent fuel pool is provided. The methodincludes: providing a spent fuel pool arranged inside a containmentvessel and an annular reservoir surrounding the containment vessel atleast partially filled with coolant water at a first temperature, thespent fuel pool and containment vessel sharing a thermally-conductivecommon wall disposed between the spent fuel pool and the annularreservoir; at least partially filing the spent nuclear fuel pool with abody of water having a surface level; submerging a spent fuel rackcontaining at least one nuclear spent fuel rod in the body of water inthe spent fuel pool; heating the water in the spent fuel pool with theat least one spent fuel rod to a second temperature higher than firsttemperature; contacting the common wall with the heated water in thespent fuel pool; transferring heat from the heated water in the spentfuel pool through the common wall to the coolant water in the annularreservoir thereby cooling the heated water in the spent fuel pool.

In another aspect, the invention can also be a passively-cooled spentnuclear fuel pool system comprising: a spent nuclear fuel poolcomprising: a body of liquid water having a surface level, at least onespent nuclear fuel rod submerged in the body of liquid water that heatsthe body of liquid water; a lid covering the spent nuclear fuel pool toform a hermetically sealed vapor space between the surface level of thebody of liquid water and the lid, the lid comprising a first lid sectionand a second lid section; and a first divider extending from the lid apartial distance into the body of liquid water to divide the vapor spaceinto a first vapor space section located between the first lid sectionand the body of liquid water and a second vapor space section locatedbetween the second lid section and the body of liquid water; and apassive heat exchange sub-system comprising: a riser conduit comprisinga first riser inlet section having a first inlet positioned within thefirst vapor space section, a second riser inlet section having a secondinlet positioned within the second vapor space section and a primaryriser section, wherein the riser conduit receives water vapor from thefirst and second vapor space sections; at least one downcomer fluidlycoupled to the primary riser section for receiving the water vapor fromthe primary riser section, the water vapor condensing within the atleast one downcomer to form a condensed water vapor; and at least onereturn conduit fluidly coupled to the at least one downcomer, the atleast one return conduit having an outlet located within the body ofliquid water for returning the condensed water vapor to the body ofliquid water.

In another aspect, the invention can be a passively-cooled spent nuclearfuel pool system comprising: a spent nuclear fuel pool comprising: abody of liquid water having a surface level, at least one spent nuclearfuel rod submerged in the body of liquid water that heats the body ofliquid water; and a lid covering the spent nuclear fuel pool to create avapor space between the surface level of the body of liquid water andthe lid; and a passive heat exchange sub-system comprising: at least oneriser conduit having an inlet located within the vapor space forreceiving water vapor from the vapor space; at least one downcomerconduit fluidly coupled to the riser conduit for receiving the watervapor from the riser conduit, the water vapor condensing within thedowncomer conduit to form a condensed water vapor; and at least onereturn conduit fluidly coupled to the at least one downcomer conduit,the return conduit having an outlet located within the body of liquidwater for returning the condensed water vapor to the body of liquidwater.

In yet another aspect, the invention can be a passively-cooled spentnuclear fuel pool system comprising: a spent nuclear fuel poolcomprising a body of liquid water having a surface level, at least onespent nuclear fuel rod submerged in the body of liquid water that heatsthe body of liquid water; a lid covering the spent nuclear fuel pool tocreate a hermetically sealed vapor space between the surface level ofthe body of liquid water and the lid; and a passive heat exchangesub-system fluidly coupled to the vapor space, the passive heat exchangesub-system configured to: (1) receive water vapor from the vapor space;(2) remove thermal energy from the received water vapor, therebycondensing the water vapor to form a condensed water vapor; and (3)return the condensed water vapor to the body of liquid water.

In a further aspect, the invention can be a method of passively coolinga spent nuclear fuel pool comprising: a) covering the spent nuclear fuelpool with a lid thereby forming a vapor space having water vapor betweenthe lid and a surface level of a body of liquid water located within thespent fuel pool; b) passively flowing the water vapor from the vaporspace through a heat exchange sub-system that removes thermal energyfrom the water vapor to form a condensed water vapor; and c) passivelyflowing the condensed water vapor from the heat exchange sub-system tothe body of liquid water.

In a still further aspect, the invention can be a method of passivelycooling a spent nuclear fuel pool comprising: a) at least partiallyfiling the spent nuclear fuel pool with a body of liquid water having asurface level; b) submerging at least one nuclear fuel rod in the bodyof liquid water, the at least one nuclear fuel rod heating the body ofliquid water; c) covering the body of a liquid water with a lid to forma hermetically sealed vapor space between the surface level of the bodyof liquid water and the lid, the lid comprising a first lid section anda second lid section; d) dividing the vapor space into a first vaporspace section located between the first lid section and the body ofliquid water and a second vapor space section located between the secondlid section and the body of liquid water, the first and second vaporspace sections being hermetically isolated from one another by adivider; e) fluidly coupling a heat exchange sub-system to the spentnuclear fuel pool, the heat exchange sub-system having a riser conduit,a downcomer conduit and a return conduit that are fluidly coupledtogether, the riser conduit having an inlet positioned within each ofthe first and second vapor space sections and the return conduit havingan outlet positioned within the body of liquid water; and wherein watervapor flows from the first and second water vapor space sections to theriser conduit and from the riser conduit into the downcomer conduit,wherein the water vapor condenses within the downcomer conduit to form acondensed water vapor, and wherein the condensed water vapor flows fromthe downcomer conduit and into the return conduit and from the returnconduit into the body of liquid water.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the exemplary embodiments of the present invention willbe described with reference to the following drawings, where likeelements are labeled similarly, and in which:

FIG. 1 is a front view of a containment enclosure for a nuclear reactorin accordance with an embodiment of the present invention;

FIG. 2 is a cross-sectional view through the containment enclosure takenalong line II-II of FIG. 1 illustrating a containment vessel at leastpartially surrounded by the containment enclosure;

FIG. 3 is a cross-sectional view taken along line III-III of FIG. 2;

FIG. 4 is a cross-sectional view taken along line IV-IV of FIG. 2;

FIG. 5 is a schematic cross-sectional view through the containmentenclosure and the containment vessel;

FIG. 6 is a close-up view of area VI of FIG. 5;

FIG. 7 is a schematic view of a generalized cross-section of a portionof the nuclear reactor containment enclosure and the containment vesselof FIG. 2 depicting a spent nuclear fuel pool and a nuclear reactortherein;

FIG. 8 is a schematic view of a cross-section of a spent nuclear fuelpool in accordance with an embodiment of the present invention;

FIG. 8A is a close-up view of area VIIIA of FIG. 8;

FIG. 9 is a schematic view of a generalized cross-section of a portionof a nuclear reactor containment enclosure and containment vessel inaccordance with another embodiment of the present invention whereby thespent nuclear fuel pool of FIG. 8 is contained within the containmentvessel.

FIG. 10 is a cross-sectional elevation view of the spent nuclear fuelpool and annular reservoir showing additional features for cooling thefuel pool;

FIG. 11 is the same view thereof with addition of a flow partition wallin the spent fuel pool;

FIG. 12 is a side elevation of a spent fuel rack configured for holdingspent nuclear fuel rods which incorporates cooling features; and

FIG. 13 is a top plan view thereof.

All drawings are schematic and not necessarily to scale.

DETAILED DESCRIPTION OF THE INVENTION

The features and benefits of the invention are illustrated and describedherein by reference to exemplary embodiments. This description ofexemplary embodiments is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. In the description of embodiments disclosed herein,any reference to direction or orientation is merely intended forconvenience of description and is not intended in any way to limit thescope of the present invention. Relative terms such as “lower,” “upper,”“horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivative thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description onlyand do not require that the apparatus be constructed or operated in aparticular orientation. Terms such as “attached,” “affixed,”“connected,” “coupled,” “interconnected,” and similar refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise. Accordingly, the disclosure expressly should not belimited to such exemplary embodiments illustrating some possiblenon-limiting combination of features that may exist alone or in othercombinations of features.

Referring first to FIGS. 1 and 2 concurrently, a nuclear reactorcontainment system 100 is illustrated in accordance with an embodimentof the present invention. The system 100 generally includes an innercontainment structure such as a containment vessel 200 and an outercontainment enclosure 300. The containment vessel 200 and thecontainment enclosure 300 are vertically elongated structures that housecertain components of a nuclear power plant such as a nuclear reactorand a spent nuclear fuel pool. The containment vessel 200 and thecontainment enclosure 300 collectively form a containmentvessel-enclosure assembly 200-300. In certain embodiments, thecontainment enclosure 300 and the containment vessel 200 may bepartially buried in the subgrade. The details of the containment vessel200 and the containment enclosure 300 are described in detail below andin International Application No. PCT/US13/42070, filed on May 21, 2013,the entirety of which is incorporated herein by reference.

In certain embodiments, the containment vessel-enclosure assembly200-300 may be supported by a concrete foundation 301 comprised of abottom slab 302 and vertically extending sidewalls 303 rising from thebottom slab 302 and forming a top base mat 304. The sidewalls 303 maycircumferentially enclose a lower portion 209 of the containment vessel200 as shown in FIG. 2 wherein the lower portion 209 of the containmentvessel 200 is positioned inside the sidewalls 303. In some embodiments,the sidewalls 303 may be poured after placement of the containmentvessel 200 on the bottom slab 302 (which may be poured and set first)thereby completely embedding the lower portion 209 of the containmentvessel 200 within the foundation 301. The foundation sidewalls 303 mayterminate below grade in some embodiments to provide additionalprotection for the containment vessel-enclosure assembly 200-300 fromprojectile impacts (e.g. crashing plane, etc.). The foundation 301 mayhave any suitable configuration in a top plan view, including withoutlimitation polygonal (e.g. rectangular, hexagon, circular, etc.).

The containment enclosure 300 may be a double-walled structure in someembodiments having sidewalls 320 formed by two substantially radiallyspaced and interconnected concentric shells 310 (inner) and 311 (outer)with plain or reinforced concrete 312 installed in the annular spacebetween the inner and outer shells 310, 311. In such embodiments, theinner and outer shells 310, 311 may be made of any suitably strongmaterial, such as for example without limitation ductile metallic platesthat are readily weldable (e.g. low carbon steel). Other suitablemetallic materials including various alloys may be used. In oneembodiment, without limitation, the double-walled containment enclosure300 may have a concrete 312 thickness of six feet or more which ensuresadequate ability to withstand high energy projectile impacts such asthat from an airliner.

XX The containment enclosure 300 circumscribes the containment vessel200 and is spaced substantially radially apart from the containmentvessel 200, thereby creating a heat sink space 313 between an outersurface 251 of the containment vessel 200 and the inner shell 310 of thecontainment enclosure 300. The heat sink space 313 may be a liquidreservoir in one embodiment such that the heat sink space 313 is filledwith a liquid such as water to create a heat sink for receiving anddissipating heat from the containment vessel 200 in the case of athermal energy release incident inside the containment vessel 200. Theheat sink can also be used to remove thermal energy from a spent nuclearfuel pool located within the containment vessel 200 as discussed in moredetail below with reference to FIGS. 7 and 9. This water-filled heatsink space 313 extends circumferentially for a full 360 degrees in oneembodiment such that the heat sink space 313 is an annular spacecircumferentially surrounding the containment vessel 200. In oneembodiment, the heat sink space 313 is filled with liquid from the basemat 304 at the bottom end 314 of the concentric shells 310, 311 of thecontainment enclosure 300 to approximately the top end 315 of theconcentric shells 310, 311 of the containment enclosure 300 to form anannular cooling water reservoir between the containment vessel 200 andthe inner shell 310 of the containment enclosure 300. This annularreservoir may be coated or lined in some embodiments with a suitablecorrosion resistant material such as aluminum, stainless steel, or asuitable preservative for corrosion protection. In one representativeexample, without limitation, the heat sink space 313 may be about 10feet wide and about 100 feet high.

In one embodiment, the containment enclosure 300 includes a steel dome316 that is suitably thick and reinforced to harden it against crashingaircraft and other incident projectiles. The dome 316 may be removablyfastened to the shells 310, 311 by a robust flanged joint. In oneembodiment, the containment vessel 200 is entirely surrounded on allexposed above grade portions by the containment enclosure 300, whichpreferably is sufficiently tall to provide protection for thecontainment vessel 200 against aircraft hazard or comparable projectileto preserve the structural integrity of the water mass in the heat sinkspace 313 surrounding the containment vessel 200. In one embodiment, thecontainment enclosure 300 extends vertically below grade to the top basemat 304.

The containment enclosure 300 may further include at least onerain-protected vent 317 which is in fluid communication with the heatsink space 313 and a head space 318 located between the dome 316 and thecontainment vessel 200 to allow water vapor to flow, escape, and vent tothe atmosphere. Thus, in certain embodiments due to the vent 317 thecontainment enclosure 300 may be considered to have an open top end. Inone embodiment, the vent 317 may be located at the center of the dome316, although the invention is not to be so limited and the vent 317 canbe otherwise located. In other embodiments, a plurality of vents may beprovided spaced substantially radially around the dome 316. The vent 317may be formed by a short section of piping in some embodiments which iscovered by a rain hood of any suitable configuration that allows steamto escape from the containment enclosure 300 but minimizes the ingressof water.

In some embodiments, the head space 318 between the dome 316 and thecontainment vessel 200 may be filled with an energy absorbing materialor structure to minimize the impact load on the dome 316 of thecontainment enclosure 300 from a crashing or falling projectile such as,for example without limitation, an airliner, a meteor or the like. Inone example, a plurality of tightly-packed undulating or corrugateddeformable aluminum plates may be disposed in part or all of the headspace 318 to form a crumple zone which will help absorb and dissipatethe impact forces on the dome 316.

In the exemplified embodiment, the containment structure 200 is anelongated vessel 202 including a hollow cylindrical shell 204 having acircular transverse cross-section, a top head 206, and a bottom head208. In certain embodiments the containment vessel 200 may be considereda thermally conductive containment vessel in that the containment vessel200 is formed of a thermally conductive material (i.e., metal or thelike as discussed below) and can be used to transfer heat from theinterior of the containment vessel 200 to the heat sink space 313. Inone embodiment, the containment vessel 200 may be made from a suitablystrong and ductile metallic plate and bar stock that is readilyweldable, such as, for example without limitation, a low carbon steel.In one embodiment, the cylindrical shell 204 of the containment vessel200 may be formed of a low carbon steel having a thickness of at leastone inch. Other suitable metallic materials that can be used for thecontainment vessel 200 include without limitation various metallicalloys and the like.

In one embodiment, the weight of the containment vessel 200 may beprimarily supported by the bottom slab 302 on which the containmentvessel 200 rests and the containment enclosure 300 may be supported bythe base mat 304 formed atop the sidewalls 303 of the foundation 301.Other suitable containment vessel 200 and containment enclosure 300support arrangements may be used. In one embodiment, the bottom of thecontainment vessel 200 may include a ribbed support stand 208 a orsimilar structure attached to the bottom head 208 to help stabilize andprovide level support for the containment vessel on the slab 302 of thefoundation 301.

Referring now to FIGS. 3 and 4 concurrently, the invention will befurther described. FIG. 3 illustrates a top cross-sectional view of thecontainment enclosure 300 and the containment vessel 200 and the heatsink space 313 therebetween and FIG. 4 illustrates a longitudinalcross-sectional view thereof. As noted above, the containment vessel 200has an inner surface 250 and an outer surface 251, the inner surface 250defining an interior cavity 260 of the containment vessel 200. In theexemplified embodiment, the containment vessel 200 has a plurality ofheat exchange fins 220 extending from the outer surface 251 of thecontainment vessel 200 and into the liquid reservoir in the heat sinkspace 313. However, the invention is not to be so limited in allembodiments and in certain other embodiments the heat exchange fins 220may be omitted. In the exemplified embodiment, the heat exchange fins220 are spaced circumferentially around the perimeter of the shell 204of the containment vessel 200 and extend substantially radially outwardsfrom the containment vessel 200 into the heat sink space 313.

Referring solely to FIG. 3, the heat exchange fins 220 will be furtherdescribed. The heat exchange fins 220, when used, serve multipleadvantageous functions including without limitation: (1) stiffening thecontainment vessel 200; (2) preventing excessive “sloshing” of water inheat sink space 313 in the occurrence of a seismic event; and (3) actingas heat transfer “fins” to dissipate heat absorbed by conduction throughthe containment vessel 200 to the environment of the heat sink space313.

Accordingly, in one embodiment to maximize the heat transfereffectiveness, the heat exchange fins 220 extend vertically forsubstantially the entire height of the heat sink space 313 covering theeffective heat transfer surfaces of the containment vessel 200 (i.e.portions not buried in concrete foundation) to transfer heat from thecontainment vessel 200 to the liquid reservoir in the heat sink space313. In one embodiment, the heat exchange fins 220 have upper horizontalends 217 which terminate at or proximate to the underside or bottom of atop portion 216 of the containment vessel 200, and lower horizontal ends218 which terminate at or proximate to the base mat 304 of the concretefoundation 301. In one embodiment, the heat exchange fins 220 may have aheight which is equal to or greater than one half of a total height ofthe shell 204 of the containment vessel 200.

The heat exchange fins 220 may be made of steel (e.g. low carbon steel)or other suitable metallic materials including alloys which are eachwelded on one of the longitudinally-extending sides to the outer surface251 of the containment vessel 200. The opposing longitudinally-extendingside of each heat exchange fin 220 lies proximate to, but is notpermanently affixed to the interior of the inner shell 310 of thecontainment enclosure 300 to maximize the heat transfer surface of theribs acting as heat dissipation fins. Thus, the non-welded side of theheat exchange fins 220 is spaced from the inner shell 310 of thecontainment enclosure 300 by a small gap. In one embodiment, the heatexchange fins 220 extend substantially radially outwards beyond the topportion 216 of the containment vessel 200. In one representativeexample, without limitation, steel heat exchange fins 220 may have athickness of about one inch. Other suitable thickness of fins may beused as appropriate. Accordingly, in some embodiments, the heat exchangefins 220 have a radial width that is more than 10 times the thickness ofthe heat exchange fins 220.

In one embodiment, the heat exchange fins 220 are oriented at an obliqueangle to the containment vessel 200. This orientation forms a crumplezone extending 360 degrees around the circumference of the containmentvessel 200 to better resist projectile impacts functioning incooperation with the outer containment enclosure 300. Accordingly, animpact causing inward deformation of the inner and outer shells 310, 311of the containment enclosure 300 will bend the heat exchange fins 220,which in the process will distribute the impact forces without directtransfer to and rupturing of the inner containment vessel 200 as mightpossibly occur with fins oriented 90 degrees to the containment vessel200. In other possible embodiments, depending on the construction of thecontainment enclosure 300 and other factors, a perpendicular arrangementof the heat exchange fins 220 to the containment vessel 200 may beappropriate.

Referring to FIGS. 3-6 concurrently, the invention will be furtherdescribed. The invention includes a passive heat exchange sub-system 340that is fluidly coupled to a spent nuclear fuel pool 600 (see FIGS. 7and 9, discussed in more detail below) that is housed within andenclosed by the containment vessel 200. The details of the operation ofthe passive heat exchange sub-system 340 will be discussed in moredetail below with reference to FIGS. 7 and 9.

In the exemplified embodiment, the passive heat exchange sub-system 340comprises, in part, at least one downcomer conduit 341, an inletmanifold 343 and an outlet manifold 344. In certain embodiments theinlet manifold 343 and the outlet manifold 344 may be omitted. In theexemplified embodiment, a plurality of the downcomer conduits 341 areillustrated being in intimate surface contact and therefore directlycoupled to the inner surface 250 of the containment vessel 200.Furthermore, in certain embodiments the inlet and outlet manifolds 343,344 may also be in intimate surface contact and directly coupled to theinner surface 250 of the containment vessel 200.

The downcomer conduits 341 may in certain embodiments be made of metalsuch as steel and be welded to the inner surface 250 of the containmentvessel 200. In the exemplified embodiment, the plurality of downcomerconduits 341 are circumferentially spaced around the circumference ofthe containment vessel 200 and extend parallel to a longitudinal axisA-A of the containment vessel 200. As illustrated in FIGS. 5 and 6, inone embodiment the downcomer conduits 341 may be comprised of verticallyoriented C-shaped structural channels (in cross section) positioned sothat the parallel legs of the channels are each seam welded to thecontainment vessel 200 for their entire height to define a sealedvertical flow conduit. Other suitably shaped and configured downcomerconduits may be provided so long as the fluid conveyed in the downcomerconduits is in thermal cooperation with the heat sink space 313 totransfer heat to the heat sink space 313 as discussed in more detailbelow.

Although illustrated and described whereby the downcomer conduits 341are coupled directly to the inner surface 250 of the containment vessel200, the invention is not to be so limited in all embodiments. Incertain embodiments the downcomer conduits 341 may be formed, partiallyor entirely, directly into the containment vessel 200 in between theinner and outer surfaces 250, 251 of the containment vessel 200. In suchembodiments, the containment vessel 200 may have a thickness that issufficient to support the downcomer conduits 341 between the inner andouter surfaces 250, 251 thereof. Thus, the downcomer conduits 341 may beducts or passageways that extend vertically through the body of thecontainment vessel 200 in between the inner and outer surfaces 250, 251thereof.

In the exemplified embodiment, each of the downcomer conduits 341 isfluidly coupled to both the inlet manifold 343 and the outlet manifold344 and extends between the inlet manifold 343 and the outlet manifold344. In the exemplified embodiment, each of the inlet and outletmanifolds 343, 344 is an annular structure that is fluidly connected toeach of the downcomer conduits 341. In the exemplified embodiment, theinlet and outlet manifolds 343, 344 are vertically spaced apart andpositioned at suitable elevations on the inner surface 250 of thecontainment vessel 200 to maximize the transfer of heat between fluidflowing vertically inside the downcomer conduits 341 and the containmentvessel 200 in the active heat transfer zone defined by portions of thecontainment vessel 200 having the external longitudinal fins 220 and/orsurrounded by the heat sink space 313. To take advantage of the liquidreservoir in the heat sink space 313 for heat transfer, the inlet andoutlet manifolds 343, 344 may each respectively be located on the innersurface 250 of the containment vessel 200 adjacent and near to the topand bottom of the heat sink space 313.

In one embodiment, the inlet and outlet manifolds 343, 344 may each beformed of half-sections of steel pipe which are welded directly to theinner surface 250 of the containment vessel 200. In other embodiments,the inlet and outlet manifolds 343, 344 may be formed of completesections of arcuately curved piping supported by and attached to theinner surface 250 of the containment vessel 200 by any suitable means.In still other embodiments, the inlet and outlet manifolds 343, 344 maybe formed directly into the containment vessel 200 in the space betweenthe inner and outer surfaces 250, 251 of the containment vessel 200. Infurther embodiments, the inlet and outlet manifolds 343, 344 may bedirectly coupled to the downcomers 341 but may be spaced from the innersurface 250 of the containment vessel 200.

In certain embodiments, some of the downcomer conduits 341 may beconnected to the inlet and outlet manifolds 343, 344 while others of thedowncomer conduits 341 may not be connected to the inlet and outletmanifolds 343, 344 so that various downcomer conduits 341 can playdifferent roles in the passive cooling of the interior of thecontainment vessel 200. Due to the coupling of the downcomer conduits341 to the inlet and outlet manifolds 343, 344 in the exemplifiedembodiment, any air, liquid or fluid that enters into the inlet manifold343 (as discussed in detail below with reference to FIGS. 7 and 9) willflow downwardly through the downcomer conduits 341, and heat will betransferred from the air, liquid or fluid flowing through the downcomerconduits 341 into the heat sink space 313 to thereby cool the air,liquid or fluid flowing through the downcomer conduits 341. Thus, thedownward pointing arrows in each of the downcomer conduits 341 depictedin FIG. 4 illustrate the direction of flow of air, liquid or fluid thatflows through the downcomer conduits 341 during passive spent nuclearfuel pool cooling operations, as discussed in more detail below withreference to FIGS. 7 and 9.

Any suitable number and arrangement of downcomer conduits 341 may beprovided depending on the heat transfer surface area required forcooling the fluid flowing through the downcomer conduits 341. Thedowncomer conduits 341 may be uniformly or non-uniformly spaced on theinner surface 250 of the containment vessel 200, and in some embodimentsgrouped clusters of downcomer conduits 341 may be circumferentiallydistributed around the containment vessel 200. The downcomer conduits341 may have any suitable cross-sectional dimensions depending on theflow rate of fluid carried by the ducts and heat transferconsiderations.

Referring now to FIG. 7, one embodiment of the interior of thecontainment enclosure 300 and the containment vessel 200 will bedescribed. The containment vessel 200 encloses and houses a nuclearreactor 500 and a spent nuclear fuel pool 600. The reactor comprising areactor vessel is disposed in a water-filled hotwell 501, whose use anddesign are well known in the art without further elaboration. Thepassive heat exchange sub-system 340 is coupled to the inner surface 250of the containment vessel 200 in the manner described above, althoughcertain conduits of the passive heat exchange sub-system 340 can beformed directly into the containment vessel 200 as discussed above. Thepassive heat exchange sub-system 340 is also fluidly coupled to thespent nuclear fuel pool 600 to passively cool the spent nuclear fuelpool 600 as discussed in detail below.

The spent nuclear fuel pool 600 comprises a peripheral sidewall 601 anda floor 602 that collectively define an interior cavity 603. In theexemplified embodiment, the peripheral sidewall 601 and the floor 602 ofthe spent nuclear fuel pool 600 are formed of concrete, although othermaterials commonly used for spent nuclear fuel pool construction can beused in other embodiments. A body of liquid water 605 having a surfacelevel 606 is positioned within the spent nuclear fuel pool 600, and morespecifically the body of liquid water 605 fills the interior cavity 603of the spent nuclear fuel pool 600. Furthermore, at last one spentnuclear fuel rod 607 is submerged in the body of liquid water 605. Theat least one spent nuclear fuel rod 607 has a high heat and thereforeheats the body of liquid water 605 within the spent nuclear fuel pool600. The passive heat exchange sub-system 340 is used to passively coolthe body of liquid water 605 within the spent nuclear fuel pool 600 toprevent the body of liquid water 605 from boiling and evaporating, whichwould result in an undesirable situation whereby the spent nuclear fuelrod(s) 607 are exposed above the surface level 606 of the body of liquidwater 605.

In the exemplified embodiment, the spent nuclear fuel pool 600 iscovered with a lid 610. Covering the spent nuclear fuel pool 600 withthe lid 610 forms a hermetically sealed vapor space 611 between thesurface level 606 of the body of liquid water 605 and the lid 610. Thevapor space 611 is an air-filled space between the surface level 606 ofthe body of liquid water 605 and the lid 610. The vapor space 611becomes filled with vapor or evaporated water from the body of liquidwater 605 as the body of liquid water 605 becomes heated by the spentnuclear fuel rod(s) 607 submerged therein. The use of the lid 610prevents the deposition of dirt and debris into the body of liquid water605 thereby reducing the need for or frequency of using a pool clean-upsystem. Furthermore, the lid 610 prevents humidity from the spentnuclear fuel pool 600 (i.e., the water vapor in the vapor space 611)from entering into the interior cavity 260 of the containment vessel 200and thereby reduces the HVAC burden in the interior cavity 260 of thecontainment vessel 200 and increases habitability of the interior cavity260 of the containment vessel 200 by operators/workers. Furthermore, incertain embodiments the lid 610 can be designed having a flat top toenable the lid 610 to serve as a working area or equipment lay down areainside of the containment vessel 200.

The lid 610 can be formed of any desired material, including withoutlimitation concrete, metal, metallic alloys, wood or the like. The lid610 need not shield radiation in all embodiments because radiationshielding, to the extent that such is necessary, is generally achievedby the body of liquid water 605. Rather, the lid 610 is intended tocreate the hermetically sealed vapor space 611 between the lid 610 andthe surface level 606 of the body of liquid water 605. Any materialcapable of achieving such a hermetically sealed vapor space 611 can beused for the lid 610.

As noted above, the passive heat exchange sub-system 340 is fluidlycoupled to the spent nuclear fuel pool 600. More specifically, thepassive heat exchange sub-system 340 is fluidly coupled to both thevapor space 611 and to the body of liquid water 605 of the spent nuclearfuel pool 600. As noted above, the passive heat exchange sub-system 340comprises the downcomer conduits 341, the inlet manifold 343 and theoutlet manifold 344. Furthermore, the passive heat exchange sub-system340, in the exemplified embodiment, also includes a riser conduit 370and a return conduit 380. As will be discussed in detail below, thepassive heat exchange sub-system 340 is configured to receive watervapor from the vapor space 611, remove thermal energy from the receivedwater vapor, thereby condensing the water vapor, and return thecondensed water vapor to the body of liquid water 605. As a result, thewater vapor does not affect the humidity inside of the interior cavity260 of the containment vessel 200 because it remains trapped in thehermetically sealed vapor space 611 and then flows through the passiveheat exchange sub-system 340 without entering into the interior cavity260 of the containment vessel 200. Furthermore, due to the flow of thewater vapor and condensed water vapor through the passive heat exchangesub-system 340, the spent nuclear fuel pool 600, and specifically thebody of liquid water 605 therein, is passively cooled.

As will be discussed in more detail below, the passive heat exchangesub-system 340 comprises or forms a closed-loop fluid flow circuit.Specifically, water vapor flows from the spent nuclear fuel pool 700(specifically from the vapor space 611 of the spent nuclear fuel pool700) into the riser conduit 370, from the riser conduit 370 into theinlet manifold 343, from the inlet manifold into the downcomers 341,from the downcomers 341 into the outlet manifold 344, from the outletmanifold into the return conduit 380, and from the return conduit 380back into the spent nuclear fuel pool 700 (specifically into the body ofliquid water 705 within the spent nuclear fuel pool 700). Thus, thepassive heat exchange sub-system 340 forms a closed-loop fluid flowcircuit that takes heated vapor water from the spent nuclear fuel pool700, cools the heated vapor water to form a cooled condensed watervapor, and reintroduces the cooled condensed water vapor back into thespent nuclear fuel pool 700 to passively cool the body of liquid water700 within the spent nuclear fuel pool 700. The details of this systemand the fluid flow through the system will be discussed in detail below.

In the exemplified embodiment, the riser conduit 370 of the passive heatexchange sub-system 340 has an inlet 371 that is located within thevapor space 611. Furthermore, the return conduit 380 of the passive heatexchange sub-system 340 has an outlet 381 that is located within thebody of liquid water 605. Thus, as the body of liquid water 605 becomesheated by the spent fuel rods 607, the vapor space 611 becomes filledwith hot vapor water. The vapor water will flow into the passive heatexchange sub-system 340 through the inlet 371 of the riser conduit 370.The vapor water will then flow upwards within the riser conduit 370 inthe direction indicated by the arrow A.

Although the downcomer conduits 341 have been described above as beingcoupled to or in intimate surface contact (i.e., conformal surfacecontact) with the inner surface 250 of the containment vessel 200, incertain embodiments the riser conduit 370 is not similarly coupled tothe containment vessel 200. Rather, it is desirable to ensure that thewater vapor that flows through the riser conduit 370 retains its thermalenergy while within the riser conduit 370 so that the water vapor doesnot cool as it rises within the riser conduit 370. By retaining thethermal energy of the water vapor while the water vapor flows throughthe riser conduit 370, thermosiphon flow can be facilitated by ensuringthat the hot water vapor rises within the riser conduit 370 and thencools within the downcomer conduits 341. Thus, in certain embodimentsthe riser conduit 370 is spaced from the inner surface 250 and othersurfaces of the containment vessel 200 so that the riser conduit 370 isnot in thermal cooperation with the heat sink (i.e., the heat sink space313). In certain embodiments, the riser conduit 370 may also include athermal insulating layer. Such a thermal insulating layer will furtherensure that the vapor water does not condense as it flows upwardlywithin the riser conduit 370 by trapping the thermal energy of the watervapor within the riser conduit 370 as the water vapor flows upwardlywithin the riser conduit 370.

However, the invention is not to be limited by the above in allembodiments and in certain other embodiments it may be desirable tocondense the water vapor as the water vapor rises within the riserconduit 370. In such embodiments the riser conduit 370 may be coupled toor in intimate surface contact with the inner surface 250 of thecontainment vessel 200. Alternatively, the thermal insulating layer maybe omitted and the water vapor may condense as it rises due to naturalthermal energy transfer and natural cooling that occurs over time due tothe temperature in the interior cavity 260 of the containment vessel 200being less than the temperature of the water vapor within the riserconduit 370.

The vapor water will continue to flow within the riser conduit 370 inthe direction of the arrow A until it is fed into the inlet manifold343. In the exemplified embodiment, the inlet manifold 343 fluidlycouples the riser conduit 370 to the one or more downcomer conduits 341.Thus, after entering into the inlet manifold 343, the vapor water willflow out of the inlet manifold 343 and into the one or more downcomerconduits 341. As noted above, the downcomer conduits 341 in certainembodiments are coupled directly to the inner surface 250 of thecontainment vessel 200. As a result, the downcomer conduits 341 are inthermal cooperation with the heat sink created by the liquid reservoirin the heat sink space 313. Due to this thermal cooperation between thedowncomer conduits 341 and the heat sink, thermal energy is transferredfrom the water vapor carried within the downcomer conduits 341 outwardlyto the heat sink (i.e., to the liquid reservoir in the heat sink space313). Specifically, the thermal energy from the water vapor istransferred to the heat sink through the one or more downcomer conduits341 and through the containment vessel 200. Thus, the heat sink space313 and the liquid reservoir therein serves as the heat sink for thedecay heat in the spent nuclear fuel pool 6000 by rejecting the heatfrom the water vapor that flows within the downcomer conduits 341 to theenvironment by natural evaporation. As the thermal energy is transferredfrom the water vapor to the heat sink, the water vapor condenses in thedowncomer conduits 341, cools, and forms a condensed water vapor.

The thermal energy transfer from the vapor water to the heat sinkfacilitates the natural, passive thermosiphon flow of the water vaporthrough the passive heat exchange sub-system 340. Specifically, the hotwater vapor rises within the riser conduit 370 and cools within thedowncomer conduits 341. The hot water vapor will continually flowthrough the riser conduits 370 and continue to passively push the watervapor through the closed-loop fluid circuit of the passive heat exchangesub-system 340.

After the water vapor condenses within the downcomer conduits 341 toform condensed water vapor (i.e., liquid water), the condensed watervapor continues to flow downwardly within the downcomer conduits 341 inthe direction of the arrows B. In some embodiments, this downward flowof the condensed water vapor can be achieved by gravity action. Thecondensed water vapor flows from the downcomer conduits 341 and into theoutlet manifold 343. The outlet manifold 344 fluidly couples thedowncomer conduits 341 to the return conduit 380. Thus, from the outletmanifold 343 the condensed water vapor flows into the return conduit380, through the return conduit 380, and out through the outlet 381 ofthe return conduit 380 into the body of liquid water 605. The condensedwater vapor mixes with the body of liquid water 605 in the spent nuclearfuel pool 600.

In some embodiments, the condensed water vapor has a temperature that islower than the average temperature of the body of liquid water 605within the spent nuclear fuel pool 600 due to the thermal energytransfer discussed above. The hot water vapor is continually removedfrom the spent nuclear fuel pool 600 and reintroduced into the spentnuclear fuel pool 600 as cooled condensed water vapor. Thus, using thepassive heat exchange sub-system 340, the spent nuclear fuel pool 600,and more specifically the body of liquid water 605 within the spentnuclear fuel pool 600, can be passively cooled by flowing the hot watervapor out of the spent nuclear fuel pool 600 and returning the cooledcondensed water vapor back into the body of liquid water 605. Thepassive heat exchange sub-system 340 facilitates thermosiphon flow ofthe water vapor as discussed above without the use of any pumps ormotors.

In certain embodiments, the air in the vapor space 611 can be partiallyevacuated (i.e., vacuumed) to a sub-atmospheric pressure so that theevaporation temperature of the body of liquid water 605 is lowered tobetween about 120° F. and 180° F., more specifically between about 135°F. and 165° F., and still more specifically to about 150° F. Evacuatingthe air in the vapor space 611 ensures that the vapor space remainsfilled with vapor water so that the thermosiphon flow of the water vaporthrough the passive heat exchange sub-system 340 can be achieved. Yet incertain other embodiments, the air in vapor space 611 may be atatmospheric or above atmospheric pressure.

Referring now to FIGS. 8 and 9, a spent nuclear fuel pool 700 will bedescribed in accordance with another embodiment of the presentinvention. Certain features of the spent nuclear fuel pool 700 aresimilar to features described above with regard to the spent nuclearfuel pool 600. Those similar features will be similarly numbered exceptthat the 700-series of numbers will be used.

The spent nuclear fuel pool 700 comprises a peripheral sidewall 701 anda floor 702 that collectively define an interior cavity 703. A body ofliquid water 705 having a surface level 706 is positioned in the spentnuclear fuel pool 700 within the interior cavity 703. At least one spentnuclear fuel rod 707 is submerged in the body of liquid water 705.Because the spent nuclear fuel rod 707 is extremely hot, the spentnuclear fuel rod 707 heats the body of liquid water 705. Thus, the bodyof liquid water 705 is, in certain embodiments, continuously cooled toremove the heat produced by the spent nuclear fuel rod(s) 707 by usingthe passive heat exchange sub-system 340, as will be discussed in moredetail below.

In the exemplified embodiment, a lid 710 covers the spent nuclear fuelpool 700 and forms a hermetically sealed vapor space 711 between thesurface level 706 of the body of liquid water 705 and the lid 710. Inthe exemplified embodiment, the lid 710 comprises a first lid section721, a second lid section 722 and a third lid section 723 thatcollectively cover the entire spent nuclear fuel pool 700. Althoughthree different lid sections are illustrated in the exemplifiedembodiment, the invention is not to be so limited in all embodiments.Thus, the lid 710 may include only two lid sections or the lid 710 mayinclude more than three lid sections in other embodiments. The number oflid sections in certain embodiments corresponds with the number ofdividers as discussed below (there will be one more lid section thanthere are dividers in certain embodiments).

Furthermore, in the exemplified embodiment a first divider 730 extendsfrom the lid 710 a partial distance into the body of liquid water 705and a second divider 731 extends from the lid 710 a partial distanceinto the body of liquid water 705. Each of the dividers 730, 731 is apartial depth wall that extends a partial depth into the body of liquidwater 705. The dividers 730, 731 can be formed of any desired materialsuch as metal, metal alloys, concrete and the like. The first divider730 extends from the lid 710 at a position between the first and secondlid sections 721, 722 and into the body of liquid water 705 and thesecond divider 731 extends from the lid 710 at a position between thesecond and third lid sections 722, 723 and into the body of liquid water705. The dividers 730, 731 may be directly coupled to the lid 710 incertain embodiments such as by welding, adhesive, fasteners or the like,or may be indirectly coupled to the lid 710 by intervening structures.Although two dividers are illustrated in the exemplified embodiment, theinvention is not to be so limited. Thus, in certain other embodimentsthere may only be one divider, or there may be more than two dividers.In certain embodiments, if one divider is used, then the lid has two lidsections, if two dividers are used, then the lid has three lid sections,if three dividers are used, then the lid has four lid sections and soon.

Each of the first and second dividers 730, 731 extends from the lid 710and into the body of liquid water 705, but not all the way to the floorof the spent nuclear fuel pool 700. In certain embodiments, each of thedividers 730, 731 extends to between approximately ⅓ and ⅕ of a depth ofthe body of liquid water 705, and more specifically to betweenapproximately ¼ of a depth of the body of liquid water 705. Thus, if thespent nuclear fuel pool 700 has a depth of forty feet, each of the firstand second dividers 730, 731 may extend to between 8 and 13 feet intothe body of liquid water 705, or more specifically to approximately 10feet into the body of liquid water 705. Of course, depths of extensionof the first and second dividers 730, 731 can be greater or less thanthat noted above and the invention is not to be limited by the depth ofextension of the dividers 730, 731 into the body of liquid water 705unless specifically recited in the claims.

Because the first and second dividers 730, 731 only extend partiallyinto the body of liquid water 705, the body of liquid water 705 is ableto flow beneath the first and second dividers 730, 731. Thus, the entirebody of liquid water 705 can flow freely within the spent nuclear fuelpool 700 without any restrictions. However, the first and seconddividers 730, 731 divide the vapor space 711 into a first vapor spacesection 724 located between the first lid section 721 and the surfacelevel 706 of the body of liquid water 705, a second vapor space section725 located between the second lid section 722 and the surface level 706of the body of liquid water 705, and a third vapor space section 726located between the third lid section 723 and the surface level 706 ofthe body of liquid water 705. Each of the first, second and third vaporspace sections 724, 725, 726 are hermetically isolated from one anotherby the dividers 730, 731 and by the lid sections 721, 722, 723 so thatwater vapor in the first vapor space section 724 cannot flow into thesecond or third vapor space sections 725, 726, water vapor in the secondvapor space section 725 cannot flow into the first or third vapor spacesections 724, 726, and water vapor in the third vapor space section 726cannot flow into the first or second vapor space sections 724, 725.

In the exemplified embodiment, a gasket 735 is coupled to each of thefirst, second and third lid sections 721, 722, 723 (see FIG. 8A). Thegasket 735 may be an annular rubber gasket or any other type of knowngasket that facilitates the creation of the hermetically sealed vaporspace sections 724, 725, 726. Thus, the spent fuel pool 700 is coveredby a set of heavy lid sections 721, 722, 723 with peripheral seals sothat the space (i.e., the vapor space sections 724, 725, 726 underneaththe lid sections 721, 722, 723) is sequestered from the ambientenvironment above the lid sections 721, 722, 723.

In the exemplified embodiment, water vapor from each of the first,second and third vapor space sections 724, 725, 726 can be introducedinto the passive heat exchange sub-assembly 340. In that regard, a riserpipe 770 (see FIG. 9) that carries the water vapor from the spentnuclear fuel pool 700 to the downcomer conduits 341 comprises a primaryriser section 771 (see FIG. 9), a first riser inlet section 772, asecond riser inlet section 773 and a third riser inlet section 774. Inthe exemplified embodiment, each of the first, second and third riserinlet sections 772, 773, 774 extends into and through the concreteperipheral sidewall 701 of the spent nuclear fuel pool 700. Thisarrangement enables each of the lid sections 721, 722, 723 to beremoved/opened without affecting the intake/return piping of the passiveheat exchange sub-system 340.

The first riser inlet section 772 has a first inlet 775 positionedwithin the first vapor space section 724, the second riser inlet section773 has a second inlet 776 positioned within the second vapor spacesection 725, and the third riser inlet section 774 has a third inlet 777positioned within the third vapor space section 726. Thus, each of thefirst, second and third inlets 775, 776, 777 is located above thesurface level 706 (i.e., the maximum water surface) of the body ofliquid water 705 within a respective one of the vapor space sections724, 725, 726. The flow of the water vapor from each of the first,second and third vapor space sections 724, 725, 726 through the passiveheat exchange sub-system 340 will be discussed in more detail below withreference to FIG. 9.

Furthermore, as discussed above the passive heat exchange sub-system 340comprises a return conduit 380. In the exemplified embodiment, thereturn conduit 380 extends through the concrete sidewall 701 of thespent nuclear fuel pool 700. The return conduit 380 terminates at anoutlet 381 that is located within the body of liquid water 705. Althoughonly one return conduit 380 is illustrated in the exemplifiedembodiment, more than one return conduit 380 can be used in otherembodiments, such as having one return conduit for each vapor spacesection.

In certain embodiments, the air in each of the first, second and thirdvapor space sections 724, 725, 726 can be partially evacuated to asub-atmospheric pressure, such as by vacuuming the air out of the vaporspaces 724, 725, 726, so that the evaporation temperature of the body ofliquid water 705 is lowered to between about 120° F. and 180° F., morespecifically between about 135° F. and 165° F., and still morespecifically to about 150° F. Evacuating the air in the vapor spacesections 724, 725, 726 ensures that the vapor space sections 724, 725,726 remain filled with vapor water so that the thermosiphon flow of thewater vapor through the passive heat exchange sub-system 340 can beachieved. Yet in certain other embodiments, the air in vapor space 611may be at atmospheric or above atmospheric pressure.

Using the inventive spent nuclear fuel pool 700, any one of the lidsections 721, 722, 723 can be separately removed from the spent nuclearfuel pool 700 as desired for installing or removing a new fuel cartridgeor fuel assembly into the spent nuclear fuel pool 700. When it isdesired to remove one of the lid sections 721, 722, 723, first thepressure within the particular vapor space section 724, 725, 726 that iscovered by the lid section 721, 722, 723 to be removed is equalized toambient. Then, the lid section 721, 722, 723 is removed. While one ofthe lid sections 721, 722, 723 is removed for fuel management activity,the other lid sections 721, 722, 723 will remain covering the spentnuclear fuel pool 700. Thus, if, for example without limitation, thefirst lid section 721 is removed, the second and third lid sections 722,723 will remain in place. Thus, the second and third vapor spacesections 725, 726 will continue to be hermetically sealed vapor spaces,and the second and third riser inlet sections 773, 774 will continue toreceive water vapor from the second and third vapor space sections 725,726 and flow the received water vapor through the passive heat exchangesub-system 340. Thus, in certain embodiments the pool cooling systemwill continue to work at all times, even during fuel managementactivity, unaided by any motors or pumps.

Referring solely to FIG. 9, flow of the water vapor through the passiveheat exchange sub-system 340 when the passive heat exchange sub-systemis fluidly coupled to the spent nuclear fuel pool 700 will be described.Because each of the first, second and third vapor space sections 724,725, 726 are hermetically isolated from one another, an inlet 775, 776,777 of one of the riser inlet sections 772, 773, 774 is positionedwithin a respective one of each of the first, second and third vaporspace sections 724, 725, 726. In the exemplified embodiment, each of thefirst, second and third riser inlet sections 772, 773, 774 converge intothe primary riser section 771 so that water vapor that flows througheach of the first, second and third riser inlet sections 772, 773, 774will converge in the primary riser section 771. The primary risersection 771 of the riser conduit 770 is fluidly coupled to the one ormore downcomers 341 by the inlet conduit 343. More specifically, theprimary riser section 771 of the riser conduit 770 is fluidly coupled tothe inlet conduit 343, and the inlet conduit 343 is fluidly coupled tothe downcomers 341.

Although in the exemplified embodiment the first, second and third riserinlet sections 772, 773, 774 converge into the primary riser section771, the invention is not to be so limited in all embodiments. Incertain other embodiments each of the first, second and third riserinlet sections 772, 773, 774 may extend separately from one of the vaporspace sections 724, 725, 726 to the inlet conduit 343. Thus, the primaryriser section 771 may be omitted and the first, second and third riserinlet sections 772, 773, 774 may not converge, but may instead eachseparately carry water vapor from the vapor space sections 724, 725, 726to the inlet manifold 343 for dispersion into the downcomers 341.

Each of the primary riser section 771 and the first, second and thirdriser inlet sections 772, 773, 774 may include a thermal insulatinglayer to prevent thermal energy from leaving the water vapor while thewater vapor is flowing within the riser conduit 770. Furthermore, eachof the primary riser section 771 and the first, second and third riserinlet sections 772, 773, 774 may be spaced apart from the inner surface250 of the containment vessel 200 to prevent the transfer of thermalenergy from the water vapor to the heat sink while the water vapor isflowing within the riser conduit 770.

As noted above, the inlet manifold 343 is in fluid communication withthe downcomers 341. Thus, the water vapor flows from the riser conduit770 upwardly to the inlet manifold 343, where the water vapor then flowsinto the downcomers 341 and downwardly within the downcomers 341. Asdiscussed above, in certain embodiments the downcomers 341 are inintimate surface contact or otherwise coupled to the inner surface 250of the containment vessel 200. Thus, as the water vapor flows within thedowncomers 341, thermal energy is transferred from the water vapor,through the downcomers 341, through the containment vessel 200 and intothe heat sink (i.e., into the liquid reservoir within the head sinkspace 313). This thermal energy transfer cools and condenses the watervapor and turns the water vapor into a condensed water vapor.

The condensed water vapor then continues to flow downwardly through thedowncomers 341 by gravity action. The condensed water vapor flows intothe outlet manifold 344, and then from the outlet manifold 344 into thereturn conduit 380. From the return conduit 380, the condensed watervapor flows through the outlet 381 and into the body of liquid water705. Thus, using the passive heat exchange sub-system 340, the body ofliquid water 705 in the spent nuclear fuel pool 700 can be passivelycooled by flowing heated vapor water out of the spent nuclear fuel pool700 and flowing cooled condensed water vapor back into the spent nuclearfuel pool 700.

In certain embodiments, the present invention can be directed to amethod of passively cooling a spent nuclear fuel pool using thecomponents discussed herein above. The operation of the system has beendiscussed in detail above, and the method is achieved by the inventivesystem. Specifically, using the system components discussed above, theinventive system can passively cool a spent nuclear fuel pool, and thusthe invention can be a method of passively cooling a spent nuclear fuelpool.

Unless otherwise specified, the components described herein maygenerally be formed of a suitable material appropriate for the intendedapplication and service conditions. All conduits and piping aregenerally formed from nuclear industry standard piping. Componentsexposed to a corrosive or wetted environment may be made of a corrosionresistant metal (e.g. stainless steel, galvanized steel, aluminum, etc.)or coated for corrosion protection.

Supplemental Non-Evaporative Cooling Features of the Spent Fuel Pool

In addition to the passive heat exchange sub-system described hereinwhich cools the spent fuel pool via evaporative cooling and naturalgravity-driven flow circuits, supplemental cooling of the body of waterin the spent fuel pool is provided by heat transfer features whichutilize convective and conductive processes to lower the temperature ofthe water, thereby not only cooling the body of water but reducing therate of evaporation. Moreover, these heat transfer features serve as apassive heat exchange back-up mechanism for the heat exchange sub-systemin the event that sub-system becomes unavailable for some reason.Therefore, cooling of spent fuel pool may continue for at least alimited period of time.

FIGS. 3 and 10-13 depict the non-evaporative based cooling features ofspent fuel pool, which will now be described. FIGS. 10 and 11 show thespent fuel pool 700 of FIGS. 7-9, but with some modifications discussedbelow. Similar elements are labeled similarly and the descriptionthereof previously provided above applies and will not be repeated infull detail here for the sake of brevity.

Referring now to FIGS. 3 and 10, the nuclear spent fuel pool 700includes a plurality of vertical peripheral sidewalls 701 and a floor702 that collectively define an interior cavity 703. Floor 702 ispreferably flat and horizontal in one embodiment. The body of water 705(liquid phase) having a surface level 706 is shown contained in thespent nuclear fuel pool 700 within the interior cavity 703. At least onespent nuclear fuel rod 707 is submerged in the body of liquid water 705.In certain embodiment, multiple spent fuel rods 707 may be stored inspent fuel racks 400 which are configured to rest on the floor 702, asfurther described herein. The racks 400 may include legs 401 designed toraise the racks off the floor and create a gap therebetween to allow thecooling water to flow beneath the racks and improve cooling.

The spent fuel pool 700 may include a lid 710 as described herein whichcovers the spent nuclear fuel pool 700 and forms a hermetically sealedvapor space 711 between the surface level 706 of the body of liquidwater 705 and the lid. The lid 710 may be a single piece (see, e.g. FIG.7), or include multiple sections such as first lid section 721, secondlid section 722, and third lid section 723 that collectively cover theentire spent nuclear fuel pool 700 (see also FIGS. 8-9). Dividers, suchas without limitation first divider 730 and second divider 731 whichextend from the lid 710 a partial distance into the body of liquid water705 as shown in FIGS. 8-9, may be provided in some embodiments tosegment the vapor space 711. Other embodiments as shown in FIGS. 10 and11 may omit the dividers. The invention is not limited by the presenceor absence of dividers.

In certain embodiments, the spent fuel pool 700 may include provisionsfor the passive heat exchange sub-system 340 already described in detailherein. Accordingly, riser conduit 370 of the passive heat exchangesub-system 340 may have an inlet 371 located within the vapor space 711,and the return conduit 380 of the passive heat exchange sub-system 340may have an outlet 381 that is located within the body of liquid water705 (see also FIG. 7). The passive heat exchange sub-system 340 operatesin the same manner described elsewhere herein.

With continuing reference to FIGS. 3 and 10, the peripheral sidewalls701 may be formed of concrete except for one wall adjacent the annularreservoir 402. In the illustrated embodiment, a portion of the metalcylindrical shell 204 of the containment vessel 200 forms a sharedvertical heat transfer wall 420 common to both the containment vesseland spent fuel pool. The heat transfer wall 420 is arranged between thespent fuel pool 700 and the annular reservoir 402 defined by the heatsink 313 disposed between the containment vessel and the inner shell 310of the containment enclosure 300 already described herein (see e.g.FIGS. 2, 3, 7, and 9 also). There are no intervening structures betweenthe spent fuel pool 700 and annular reservoir 402 other than the heattransfer wall 420. Accordingly, the body of water 705 in the spent fuelpool wets an interior surface 421 of the heat transfer wall and theliquid coolant (water) impounded in the annular reservoir 402 wets anexterior surface 422 of the heat transfer wall. Advantageously, thisallows heat to be conducted through the heat transfer wall 420 betweenthe higher temperature body of water 705 in the spent fuel pool 700(heated by the spent fuel rods 707) and the lower temperature water inthe reservoir 402. In addition to the evaporative losses from the bodyof water 705 which cools the water in the spent fuel pool, the heattransfer wall 420 also acts to cool the water in the pool via conductiveheat transfer. This provides a dual cooling mechanism for the spent fuelpool 700 for effective passive heat dissipation.

Heat transfer wall 420 may have an arcuate shape in top plan view (see,e.g. FIG. 3) being formed from a portion of the cylindrical shell 204 ofthe containment vessel 200. The heat transfer wall 420 preferablyextends vertically from the floor 702 of the spent fuel pool 700 to atleast the surface level 706 of the body of water 705 in the spent fuelpool 700, alternatively to approximately the lid 710 or higher. Thisensures that portion of the spent fuel pool containing the body of water705 will benefit from the conductive cooling action of the heat transferwall. The containment vessel cylindrical shell 204, and heat transferwall 700 comprising a portion thereof, may be made of any suitableconductive metal with sufficient structural strength such as withoutlimitation carbon or low alloy steel, or other metals and alloys.

In operation, the water adjacent the spent fuel racks 400 and spent fuelrods 707 therein is heated via decay heat. The density of the heatedwater decreases, thereby causing it to rise in the spent fuel pool 700towards the body of water surface defined by surface level 706. Aportion of the heated water near the top of the body of water 705contacts the inner surface 421 of the heat transfer wall 420 and iscooled by the relatively cooler heat transfer wall (wetted on the outersurface 422 by the colder water in the annular reservoir 402). Thiscauses the now cooler and denser water to sink towards the floor 702.This is sometimes referred to as the “chimney effect.” The now cooledwater flows back towards the fuel racks 400 where it is heated again andrepeats the cycle to form a first recirculating flow pattern P1 as shownby the directional flow arrows.

To increase the chimney effect and flow pattern P1 produced in the spentfuel pool 700, a first vertical flow partition wall 430 may be disposedin the spent fuel pool 700 between a spent fuel rack storage area 403and heat transfer wall 420 as shown in FIGS. 3 and 11. The flowpartition wall 430 is spaced apart from the heat transfer wall 420 andspent fuel storage racks 400. The flow partition wall improves thecooling performance of the heat transfer wall 420. The partition wall430 may be formed of metal plate, preferably a corrosion resistancemetal such as stainless steel in some non-limiting embodiments. Othermaterials such as concrete may be used in certain embodiments. In oneconfiguration, partition wall 430 may have an arcuate shape in top planview to complement the shape of the containment vessel shell 204 andheat transfer wall 420. Other shapes may be used.

In operation, the flow partition wall 430 creates a recirculating flowpattern P1 in the spent fuel pool wherein the heated water flows upwardfrom the spent fuel rack 400 along a first side of the flow partitionwall, over the flow partition wall, downward along a second side of theflow partition wall contacting the thermally-conductive heat transferwall 420, and under the flow partition wall back towards the spent fuelrack.

The height of the flow partition wall 430 may be adjusted based on theincrease in chimney effect required. The partition wall 430 isconfigured to allow the recirculating water in the body of water 705 toflow beneath the wall as shown. Accordingly, the wall 430 may besupported from the sidewalls 701 of the spent fuel pool 700 so that thebottom of the wall is spaced vertically apart from the floor 702, flowopenings may be formed in the bottom of the wall adjacent to the floor,or vertical standoffs/supports may be attached to the wall to raise thebottom of the wall off the floor. Any suitable method for enabling flowbeneath the wall may be used and is not limiting of the invention.

It will be noted that in some embodiments, a flow partition wall 430 maynot be used as shown in FIG. 10.

Referring now to FIGS. 3, 10, and 11, a chimney effect is created withinthe water filled annular reservoir 402 due to the changing densities ofthe coolant water occurring in a similar manner to the water in thespent fuel pool 700 as described above. The coolant water adjacent theheat transfer wall 420 and inner surface 422 formed thereon is heated asheat is conducted through the heat transfer wall to the reservoir 402from the body of water 705 in the spent fuel pool 700. The coolant waterin the annular reservoir adjacent the heat transfer wall 420 isinitially at a temperature lower than the temperature of the wall andheated body of water 705 in the spent fuel pool 700. The heated lessdense water in the annular reservoir 402 rises and cools due toevaporative losses to atmosphere at the water level. The cooled lessdense coolant water which is now near the surface of the coolant waterin the reservoir sinks back towards the base mat 304. This creates asecond recirculating flow pattern P2 in the annular reservoir 402 due tothe chimney effect.

In some embodiments, a vertical flow partition plate 440 may be disposedin the annular reservoir 402 between the containment vessel shell 204and inner shell 310 of the containment enclosure 300 to increase thechimney effect and cooling performance in the heat sink 313. Partitionplate 440 is preferably disposed at least adjacent to the heat transferwall 420. This partition plate 440 may preferably be formed of a metalplate in some embodiments or other materials. In one configuration,partition plate 440 may have a substantially arcuate shape in top planview to conform to the annular shape of the annular reservoir 402. Thepartition plate 440 may be located at the mid-section of the annularreservoir 402 between the heat transfer wall 420 (and containment vesselshell 204) and the inner shell 310 of the containment enclosure 300.Preferably, the partition plate 440 has an angular extent or width atleast coextensive with the angular extent or width of the heat transferwall 420 to maximize heat removal from the spent fuel pool 700.Accordingly, partition plate 440 may extend less than 360 degreesthroughout the annular reservoir 402 in some embodiments. In otherembodiments, the partition plate 440 may extend a complete 360 degreesthroughout the entire annular reservoir 402 forming an annular shapedflow structure.

In operation, the flow partition plate 440 creates a recirculating flowpattern in the annular reservoir 402 wherein the heated coolant waterflows upward along a first side of the flow partition plate, over theflow partition plate, downward along a second side of the flow partitionplate, and under the flow partition plate back towards thethermally-conductive common wall.

The height of the flow partition plate 440 in the annular reservoir 402may be adjusted based on the increase in chimney effect required. Thepartition plate 440 is configured to allow the recirculating water inthe body of water 705 to flow beneath the wall as shown. Accordingly,the plate 440 may be supported from any available structure within theannular reservoir 402 such as the fins 220 so that the bottom of thewall is spaced vertically apart from the base mat 304. Alternatively,vertical standoffs/supports may be attached to the plate 440 to raisethe bottom of the wall off the base mat 304, or flow openings may beformed in the bottom of the plate 440 adjacent to the base mat. Anysuitable method for enabling flow to pass beneath the wall may be usedand is not limiting of the invention.

As best shown in FIG. 3, the flow partition plate 440 may be comprisedof a plurality of individual segments 442 which collectively form thepartition wall structure. The segments 442 may each be mounted betweenpairs of the adjacent circumferentially spaced apart heat exchange fins220 which serve to support the plate 440. Depending on thecircumferential spacing of the fins 220, the segments 442 in certainembodiments may each be comprised of flat metal plates welded orotherwise attached between the fins which collectively approximates anarcuately shape in top plan view (hence the shape may be referred to assubstantially arcuate). In other embodiments, the segments 442 maycomprise arcuately curved plates attached between the fins.

The segments 442 form a plurality of or wedge shaped isolated pie flowregions 441 between the fins. Flow through-holes may be provided whichextend completely through the fins 220 in some embodiments to allowmixing of the flow and reservoir coolant water between these regions.Alternatively, the fins 220 may not extend all the way to the base mat304 to allow flow to mix between the regions 441. In certainembodiments, fins 220 may not be provided in the annular reservoir 402in which case the partition plate 440 will be supported from the basemat 340 and upper portions of the plate 440 may be braced by lateralstruts attached to the cylinder shell 204 of the containment vessel 200and/or inner shell 310 of the containment enclosure 300 for rigidity.

It will be appreciated that the decay heat released by the spent fuelrods 707 in the spent fuel pool 700 is the motive force that drives boththe natural circulation and the passive cooling in both the spent fuelpool and the annular reservoir 402.

According to another aspect of the invention, each individual spent fuelrack 400 is further configured to facilitate and increase heatdissipation from the spent fuel rods for effective cooling. FIGS. 12 and13 are side elevation and top plan views respectively of a plurality offuel racks 400 which may be grouped together in close proximity on thefuel rack storage area 403 portion of the spent fuel pool floor 702.

Referring to FIGS. 11-13, each spent fuel rack 400 includes an elongatedbody comprised of a plurality of hollow tubes 407, a top plate 405, andbottom plate 406. The tubes 407 include open tops 403 which allowinsertion and storage of spent fuel rods in each tube. The bottoms 404of the tubes may be closed except for a drainage hole to enable removalof water from the tubes when the racks are lifted out of the spent fuelpool 700. In one embodiment configuration, the tubes 407 may haverectilinear transverse cross sections forming square or rectangular tubeshapes in top plan view. This allows dense packing of the tubes in eachrack to maximize spent fuel rod capacity. A plurality of legs 401 areattached to the bottom plate 406 to raise the racks off the spent fuelpool floor 702 to permit cooling water in the pool to flow beneath thebottom plate 406 for improved cooling.

The tubes 407, top plate 405, and bottom plate 406 may be formed ofmetal to efficiently conduct heat away from the spent fuel rods 707. Insome embodiments, the tubes may be formed of a metal-matrix compositematerial, such as a discontinuously reinforced aluminum/boron carbidemetal matrix composite material or boron impregnated aluminum forneutron absorption.

To maximize cooling of the racks 400 and spent fuel rods 707 containedin the tubes 407, the top and bottom plates 405, 406 may each extendlaterally beyond the tubes on all four sides of the rack by a distance408 forming peripheral extension portions 410. The extension portions410 are bare exposed metal and provide additional heat transfer surfacearea for increasing heat removal from the spent fuel racks. An exampleof a fuel rack with such an extended bottom plate is disclosed incommonly owned U.S. patent application Ser. No. 14/367,705, which isincorporated herein by reference in its entirety.

In one embodiment, the top plate 405 may be configured as a perimeterframe allowing access to the open tops of the tubes. By contrast, thebottom plate 406 may cover the entire bottom area of the tubes forming afloor for each tube. The top and bottom plates 405, 406 of each spentfuel rack 400 may have the same lateral width and length (in top planview) to allow multiple racks to be efficiently abutted when emplaced onthe fuel rack storage area 403 of the spent fuel pool floor 702. Theextension portions 410 of the racks further create horizontal gaps 409between adjacent spent fuel racks 400 to allow the cooling water in thespent fuel pool 700 to flow between the racks for additional cooling.

While the invention has been described with respect to specific examplesincluding presently preferred modes of carrying out the invention, thoseskilled in the art will appreciate that there are numerous variationsand permutations of the above described systems and techniques. It is tobe understood that other embodiments may be utilized and structural andfunctional modifications may be made without departing from the scope ofthe present invention. Thus, the spirit and scope of the inventionshould be construed broadly as set forth in the appended claims.

What is claimed is:
 1. A passively cooled spent nuclear fuel poolsystem, the system comprising: a containment vessel comprising athermally conductive cylindrical shell formed of metal; an annularreservoir surrounding the cylindrical shell of the containment vessel,the annular reservoir holding a liquid coolant to form a heat sink; anda spent fuel pool disposed inside the containment vessel, the fuel poolcomprising: a floor and a first peripheral sidewall extending upwardsfrom the floor that collectively define an interior cavity; a body ofwater disposed in the interior cavity and having a surface level, thewater being in contact with the first peripheral sidewall; and at leastone spent nuclear fuel rod submerged in the body of water that heats thebody of water; wherein the first peripheral sidewall of the fuel pool isformed by a portion of the cylindrical shell of the containment vesseladjacent to the spent fuel pool which defines a shared common heattransfer wall, the heat transfer wall operable to transfer heat from thebody of water in the spent fuel pool to the heat sink for cooling thebody of water.
 2. The system according to claim 1, wherein the annularreservoir contains water as the liquid coolant having a lowertemperature than the body of water in the spent fuel pool.
 3. The systemaccording to claim 1, wherein the heat transfer wall has an arcuateshape in top plan view.
 4. The system according to claim 1, furthercomprising a vertically oriented flow partition plate disposed in atleast a portion of the annular reservoir adjacent the heat transferwall, the flow partition plate spaced radially apart from the heattransfer wall and configured to define a first convective flow path thatinduces natural gravity circulation of the liquid coolant in the annularreservoir.
 5. The system according to claim 4, wherein the flowpartition plate has substantially arcuate shape in top plan view.
 6. Thesystem according to claim 4, wherein the flow partition plate includes abottom spaced above a base mat of the annular reservoir and a top spacedapart below a top end of the annular reservoir such that a liquidcoolant circulation flow path is formed over and under the flowpartition plate.
 7. The system according to claim 6, wherein heatedliquid coolant in the annular reservoir circulates vertically upwardalong the heat transfer wall between the flow partition plate,horizontally outward over a top of the flow partition plate, verticallydownward along a side of the flow partition plate opposite the heattransfer wall, and horizontally inward beneath at least a portion of theflow partition plate back towards the heat transfer wall to complete thefirst convective flow path.
 8. The system according to claim 7, whereinwater in the spent fuel pool cooled by exchanging heat through the heattransfer wall to the annular reservoir circulates vertically downwardalong the heat transfer wall in an opposite direction to the heatedliquid coolant in the annular reservoir.
 9. The system according toclaim 8, further comprising a vertically oriented flow partition walldisposed in the spent fuel pool between a spent fuel rack storage areaon the floor and the heat transfer wall, the flow partition wallconfigured to define a second convective flow path that induces naturalgravity circulation of the body of water in the fuel pool.
 10. Thesystem according to claim 9, wherein the flow partition wall includes abottom spaced above the floor of the spent fuel pool and a top spacedbelow the surface level of the body of water such that a watercirculation flow path is formed over and under the flow partition wall.11. The system according to claim 9, wherein the flow partition wall inthe spent fuel pool has an arcuate shape in top plan view.
 12. Thesystem according to claim 10, wherein the flow partition wall spanshorizontally between opposing second and third peripheral walls formedof concrete which intersect the heat transfer wall.
 13. The systemaccording to claim 1, further comprising a vertically oriented flowpartition wall disposed in the spent fuel pool between a spent fuel rackstorage area on the floor and the heat transfer wall, the flow partitionwall configured to define a convective flow path that induces naturalgravity circulation of the body of water in the fuel pool.
 14. Thesystem according to claim 13, wherein the flow partition wall isdisposed proximally to the heat transfer wall.
 15. The system accordingto claim 4, further comprising a plurality of heat exchange finsextending radially outwards from cylindrical shell of the containmentvessel into the annular reservoir, and wherein the flow partition plateis supported between a pair of the heat exchange fins.
 16. The systemaccording to claim 15, wherein the flow partition plate is comprised ofa plurality of segments each attached between a pair of heat exchangefins.
 17. The system according to claim 1, wherein the annular reservoiris vented to atmosphere for cooling the liquid coolant via evaporativeloss.
 18. The system according to claim 1, wherein the annular reservoiris formed between the cylindrical shell of the containment vessel and acylindrical shell of a containment enclosure surrounding the containmentvessel.
 19. A passively cooled spent nuclear fuel pool system, thesystem comprising: a containment vessel comprising a thermallyconductive cylindrical shell formed of metal; an annular reservoirsurrounding the cylindrical shell of the containment vessel, the annularreservoir holding a liquid coolant to form a heat sink; and a spent fuelpool disposed inside the containment vessel, the fuel pool comprising: afloor formed of concrete; a plurality of concrete peripheral sidewallsextending upwards from the floor that collectively define an interiorcavity configured for holding at least one spent fuel rack comprising aplurality of nuclear spent fuel rods; a shared common heat transfer wallformed by a portion of the metal cylindrical shell of the containmentvessel and arranged between the spent fuel pool and the annularreservoir; a body of water disposed in the interior cavity and having asurface level, the water being in contact with the common heat transferwall; and the spent nuclear fuel rods submerged in the body of wateroperable to heat the body of water; a vertically oriented flow partitionplate disposed in a portion of the annular reservoir adjacent to thecommon heat transfer wall, the flow partition plate spaced radiallyapart from the common heat transfer wall and configured to define afirst convective flow path that induces natural gravity circulation ofthe liquid coolant in the annular reservoir along the common heattransfer wall; wherein the common heat transfer wall is operable toconductively transfer heat from the body of water in the spent fuel poolto the heat sink for cooling the body of water.
 20. The system accordingto claim 19, wherein the flow partition plate is configured to create aliquid coolant circulation flow path over and under or through a bottomportion of the flow partition plate.
 21. The system according to claim20, further comprising a vertically oriented flow partition walldisposed in the spent fuel pool between the at least one spent fuel rackand the heat transfer wall, the flow partition wall configured to definea convective flow path that induces natural gravity circulation of thebody of water in the fuel pool along the common heat transfer wall. 22.The system according to claim 21, wherein the flow partition wall isconfigured to create a liquid coolant circulation flow path over andunder or through a bottom portion of the flow partition wall.